Cavity resonators for microwaves coupled together by variable capacitance diode



Jan. 21, 1969 A. KACH 3,423,701

CAVITY RESONATORS FOR MICROWAVES COUPLED TOGETHER BY VARIABLE CAPACITANCE DIODE Filed Aug. 11. 1966 H/(memaz: Marma a/mg ll i 1 (Me/454: awe/mm. 0/005) l i INVENTOR- 41/950 Maw BY rrmez. RAT/l ATTORNEY United States Patent 3,423,701 CAVITY RESONATORS FOR MICROWAVES COUPLED TOGETHER BY VARIABLE CA- PACITAN CE DIODE Alfred Kach, Nussbaumen, Switzerland, assignor to Patelhold Patentverwertungsund Elektro-Holding AG, Glarus, Switzerland Filed Aug. 11, 1966, Ser. No. 571,828 US. Cl. 333-83 Claims Int. Cl. H01p 7/06 ABSTRACT OF THE DISCLOSURE A cavity resonator microwave transmission device comprises a main resonant cavity having a lateral recess forming a coupling means for feeding microwave energy to said main cavity through a coaxial feeder. The coaxial feeder has an inner conductor projecting through the recess and into an auxiliary cavity forming a part of the recess. Control of the coupling is effected by varying the electric capacity between the inner conductor and the auxiliary cavity.

The present invention relates to microwave cavity resonators, more particularly, though not limitatively, resonators of the type according to my copending patent application Ser. No. 417,367, filed on Sept. 10, 1964, now Patent No. 3,278,868, entitled Cavity Resonator, having structurally embodied therein a voltage-dependent capacitor, such as a variable capacitance diode also known as a varactor, to provide a harmonic frequency multiplier or generating device.

There is described by said copending application a cavity resonator structure designed to act as an impedance matching or transforming device and including a coaxial feeding element for the impression of microwave energy upon the resonant cavity of the device via a shallow external recess or niche extending laterally from said cavity. Both said cavity and recess are fitted with suitable tuning means, such as in the form of a tuning plug or the like, to enable both a tuning adjustment and control of the transformation ratio, respectively, for the transmission of microwave energy through the device.

In a cavity resonator of this type embodied in a harmonic frequency mutliplier, the inner conductor of the coaxial feeding element, projecting into or traversing said recess, advantageously takes the form of or serially includes the semiconductor or the like capacitance diode (varactor), as shown in said copending application, the disclosure of which is hereby embodied in the instant application by reference.

Frequency multipliers utilizing variable-capacitance diodes are largly afilicted by spurious responses or interference effects which have been variously reported and described in the literature (see for example Avoiding Spurious Responses in Varactor Harmonic Generators, in Electronic Design, September 1964 issue, page 36).

The type and extent of the interference effects depend, among other causes, on the design of the multiplier circuit or structure, on the operating conditions or adjustments of the device, and on the character of the negative bias voltage of the variable capacitance diode. In this connection, attention should be drawn to the fact that the tuning adjustment of such multipliers is not fixed or constant to result in stable operating conditions, but depends both on the operating frequency as well as on the RF (high frequency or microwave) power being transmitted through the cavity. If the RF power fluctuates or varies, the variable bias voltage of the diode and in turn the effective diode capacitance at any particular time will be affected accordingly.

While the resultant drift of the tuning adjustment need not always become evident as a visible instability of the RF power transmission, the power fluctuations occurring may lie within regions of for example 20 to 30 db below the normal power output level, whereby to affect the fine structure or shape of the RF oscillations.

As will be understood, a frequency multiplier exhibiting interference effects to the extent mentioned is unsuitable, if not useless, for practical communications or signal transmission purposes.

Accordingly, an important object of the present invention is the provision of improved means in connection with a cavity resonator device of the referred to type, whereby spurious responses or interference effects as mentioned are substantially eliminated or reduced to a practical minimum and the stability of the operation greatly enhanced, compared with the previous arrangement as shown by the above mentioned copending application.

The invention, both as to the foregoing and ancillary objects, as well as novel aspects thereof, will be better understood from the following detailed description, taken in conjunction with the accompanying drawing forming part of this specification and wherein:

FIG. 1 is a vertical sectional view of a cavity resonatorvariable capacitance type harmonic frequency multiplier embodying the improved feature according to the invention;

FIG. 2 shows an equivalent circuit diagram of FIG. 1, explanatory of the function and operation of the invention;

FIG. 3 is a fragmentary view of FIG. 1, showing a modification thereof; and

FIG. 4 is a further fragmentary view of FIG. 1, showing a further improved feature of the invention.

Like reference characters denote like parts in the different views of the drawing.

With the foregoing objects in view, the invention, according to one of its aspects, involves the provision of a cavity resonator according to the copending application, especially for use in a variable capacitance type frequency multiplier, whereof the capacitance diode is secured by way of one of its end caps to the free end of an adjusting plug or plunger, forming together with the diode the inner conductor of the coaxial feeding element, the niche or recess of the main resonant cavity forming on its surface opposite to said plug or diode a further recess or cavity whereof the internal surface forms one plate of an auxiliary tunable capacitor with the free second end cap of said diode constituting the cooperating plate or electrode of said capacitor.

As a conesquence, there is provided by the invention an adjustable auxiliary (trimmer) capacity in series with the capacity of the varactor or the like voltage-dependent capacitor to enable are adjustment or variation of the transmission characteristics of the device, in such a manner as to maintain maximum output efficiency at the desired harmonic or other operating frequency being transmitted, substantially free from the effects of spurious responses or interferences mentioned, as will become further apparent as the description proceeds in reference to the drawing.

Referring more particularly to FIGS. 1 and 2, the cavity resonator shown comprises a preferably circular cylindrical cavity 1 bounded by a bottom portion or casing 3 and a top or cover plate 2. A conventional tuning plug 4, corresponding to the tuned circuit L C of FIG. 2, is ar: ranged in the center of the base of the casing 3. A coupling loop 5, corresponding to the induction coil L in the equivalent circuit of FIG. 2, is connected to the coaxial cable 6 which serves to transmit the electromagnetic output energy from said cavity to a utilization or further transmission circuit or line. Projecting laterally outwardly from the external surface of the resonant cavity 1 is a lateral niche-like recess 8 arranged with its base surfaces parallel to the base surfaces of the main cavity 1. An additional tuning plug 10, movable perpendicularly to said base surfaces, is provided for the recess 8 for the adjustment of the coupling coefi'icient of the cavity with its input circuit, in the manner described in greater detail in the copending application.

A cartridge-shaped variable capacitance diode 11 (varactor), as represented by C in FIG. 2, is removably secured by way of one of its end caps 12 to the free end of a tuning plunger 9, for example by means of a well-known spring clamp or prong, said plunger being axially adjustably mounted in the wall of the input cavity 15 and constructed in the form of a conventional tuning plug. The recess 8 communicates with an auxiliary cavity 7 extending outwardly from one of its base surfaces, at a point opposite to the plunger 9 and remote from the transition region between the cavity 1, recess 8, and said auxiliary recess being formed by an extension of the cover 2 of the main cavity 1, in the manner more clearly seen from the drawing. The internal surface of the auxiliary cavity 7 and the free second end cap 13 of the capacitance diode 11 thus form the plates or electrodes of a tunable auxiilary capacitor, adjustable by axial displacement of the plunger 9 and represented by thre capacitor C in the substitute diagram of FIG. 2. As a consequence, the auxiliary capacity C is efiectively in series with the capacity C of the capacitance diode 11.

A strip-like resistance coating 14, corresponding to resistance R in FIG. 2, is applied to the central ceramic body of the diode 11, to provide the variable bias voltage of the diode, the ends of said resistance being arranged to sufiiciently overlap the end caps 12 and 13 of the diode to ensure a close and efficient contact connection therewith.

There is thus provided a harmonic frequency multiplier of the type more clearly described by the copending application and which further comprises, in the example shown, an input resonator 15 including an input coupling loop 16, corresponding to induction coi-l L of FIG. 2., and tuning plug 18, corresponding to the tuned circuit L C of FIG. 2. The plunger 9 acts, together with the capacitance diode 11, both as the output coupling element (L of FIG. 2) of the resonator 15 tuned to the fundamental frequency f on the one hand, and as an input coupling element (L FIG. 2) of the cavity resonator 1, tuned to the frequency nf and represented by tuned circuit L C in FIG. 2, where n represents the frequency multiplication factor of the harmonic multiplier. The coupling between the inductances L and L is varied by the adjustment of the auxiliary tuning plug 10, in the manner described in the copending application.

In an arrangement of the foregoing type, FIG. 1, if C is tuned by the adjustment of the plunger 9, the capacity C between the inner and outer conductors of the coaxial coupling member, is also varied in the same sense. In practice it has been found that a close tracking can be achieved in certain cases between the two variations, that is, between the adjustments of C and C respectively. In cases Where such tracking is difficult or impossible to achieve, :a special trimmer plug may be fitted in the outer conductor opposite to the enlarged portion 19 of the plunger 9.

In the equivalent circuit diagram, FIG. 2, the capacitor C and the variable coupling between L and L provide two degrees of freedom for the tuning of the cavity 1 and matching of the multiplier output with the load impedance, respectively. Furthermore, the capacities C and C provide two further degrees of freedom or adjustments for the tuning to resonance of the input cavity and matching of the load impedance, transferred at fundamental frequency via the capacitance diode, with the internal impedance of the supply source exiting the cavity via coaxial cable 17 and coupling loop 16. The currents of the idle or undesired harmonics, which may be present between the fundamental frequency f and the useful harmonic frequency nf are substantially short-circuited by the capacity C and the inductance L both of which offer a relatively low impedance to such frequencies.

The multiplier may be set to maximum efficiency by varying the capacity C or adjusting the plug 9, respectively. Experiments have shown that the interferences mentioned may be almost completely removed in this manner. The optimum value of the capacity to which C is set usually lies between the values C representing the diode capacity at zero bias voltage, and C representing the diode capacitance at breakdown negative bias voltage. The tuning range of the capacity C should accordingly be such as to include the values C and C Variants of the invention diitering from FIG. 1 may include means to enable the capacities C and C to be adjusted independently of one another. To this end, the plunger 9, FIG. 3, may contain, in axially slidable adjusting relation therewith, a further tuning plunger or plug 20 having a head 21 and carrying a clamp for securing the diode 11 to said head via a suitable contact arrangement. Plunger 19 then serves to vary C while the plunger 20 serves to vary C respectively. Alternatively, the inner free end or cap of the diode may be fitted with interchangeable trimming elements of various sizes for the purpose of coarsely setting C in a manner readily understood. Finally, C may be varied by varying the inner dimensions of the auxiliary cavity 7, such as by means of a further adjustable plug or plunger 22, FIG. 4, mounted in the outer wall of the extension or auxiliary cavity 7. In such a case, the plunger 9 could be fixed and C varied by means of the trimmer plug 19, FIG. 1.

While the invention has been described with reference to a varactor-type harmonic frequency multiplier with the object to suppress or minimize spurious responses or interference, as well as to generally improve the efiiciency of the multiplier, the auxiliary adjustable capacitance C forming the main subject of the present improvements may be advantageously embodied in other cavity devices or systems, such as in parametric amplifiers, up-converter and the like utilizing a capacitance diode or the like, or in simple cavity-type impedance transformer or transmission devices as shown in the copending application, to improve the stability and efficiency of the microwave transmission.

It is to be understood, therefore, that variations and modifications, as well as the substitution of equivalent parts or elements for those shown herein for illustration, may be made Without departing from the broader purview and spirit of the invention as set forth in the appended claims. The specification and drawing are accordingly to be regarded in an illustrative rather than in a restrictive sense.

I claim:

1. A cavity resonator microwave device comprising in combination:

( 1) a main resonant cavity bounded by a pair of opposed end walls connected by a side Wall,

(2) a niche-like recess communicating with said cavity and extending laterally outwardly from said side wall, said recess having a dimension in the direction of the axis of said end walls being substantially less than the spacing distance between said walls,

(3) means to provide an auxiliary cavity communicating with and extending outwardly from said recess at a point remote from the transition region between said main cavity and said recess,

(4) a coaxial coupling member for feeding microwave energy to said main cavity via said recess having an outer conductor and an inner conductor extending through said recess and projecting into said auxiliary cavity, to form a capacitor with the end portion of said inner conductor and the inside wall of said auxiliary cavity as capacitative electrodes, and

(5) means to adjust the relative spacing of said member and said auxiliary cavity, to vary the capacitance of said capacitor.

2. In a cavity resonator device as claimed in claim 1, said last means being comprised of means to vary the projection of said member into said auxiliary cavity.

3. In a cavity resonator device as claimed in claim 1, said last means being comprised of means to vary the outward extension of said auxiliary cavity.

4. In cavity resonator microwave transmission device comprising in combination:

(1) a main resonant cavity bounded by a pair of 0pposed end walls connected by a side wall,

(2.) a niche-like recess communicating with said cavity and extending laterally outwardly from said side wall, said recess having a dimension in the direction of the axis of said Walls being substantially less than the spacing distance between said walls,

(3) means to provide an auxiliary cavity communicating with and extending outwardly from said recess in a direction substantially parallel to the axis of said walls and at a point remote from the transition region between said main cavity and said recess,

(4) a coaxial coupling member for feeding micro- Wave energy to said main cavity via said recess, said member having an outer conductor and an inner conductor extending through said recess and projecting into said auxiliary cavity, to form a capacitor with the end portion of said inner conductor and the inside wall of said auxiliary cavity as capacitative electrodes,

(5) a voltage-dependent capacitor serially and structurally embodied in said inner conductor, and

(6) means to adjust the relative spacing of said member and said auxiliary cavity, to vary the capacitance of said capacitor.

5. In a cavity resonator device as claimed in claim 4 being operated as a harmonic frequency multiplier, in which said voltage-dependent capacitor consists of a re versely biased junction diode.

6. In a cavity resonator device according to claim 4 being operated as a harmonic frequency multiplier with said main cavity tuned to a harmonic of the frequency of microwave energy impressed upon said coupling member, first and second adjustable tuning means, respectively, for said main cavity and said recesss, said second tuning means being located adjacent to the transition region between said main cavity and said recess.

7. In a cavity resonator device as claimed in claim 6, said adjustable tuning means consisting of a pair of tuning plugs extending, respectively, into said main cavity and said recess.

8. In a cavity resonator device as claimed in claim 4, in which said last means is comprised of means to vary the projecting distance of said member into said auxiliary cavity.

9. In a cavity resonator as claimed in claim 4, in which said last means is comprised of means to vary the outward extension of said auxiliary cavity.

10. In a cavity resonator as claimed in claim 4, wherein the inner conductor of said member forms a variable capacitor with said outer conductor, and said last means is comprised of a separate end portion of said inner conductor, and means to adjust said end portion relative to said inner conductor, to variably project into said auxiliary cavity.

References Cited UNITED STATES PATENTS 3,278,868 10/1966 K'aich 333-83 HERMAN KARL SAALBACH, Primary Examiner.

LOUIS ALLAHUT, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,423,701 January 21, 1969 Alfred Kach It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading to the printed specification, line 5, "Alfred Kach" should read Alfred Kach Signed and sealed this 7th day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR. 

